I believe we have very strong evidence, and the words of Thomas Hunter, to confirm that fluid steel tubes used by Hunter Arms (with some exceptions ie Whitworth and Krupp) were sourced from Belgium and not from a U.S. steel works
https://docs.google.com/document/pub?id=17ixogftgITEblNUWtmFBv96ZvgjK6eFell8GsAWd-KI What has
not been explained is why barrels are stamped with the SB&Co of Sanderson Bros. Steel, Syracuse.
A 1925 Long Range Field with SB&Co F (unknown - there was no 'F' Crucible steel work)
A 1926 Eagle with Nitro Steel stamped over SB&Co T (there was also no 'T' Crucible works but Tacony Iron Works, PA is possible)
I was recently sent images from a 1909 F grade hammer gun with
TWIST barrels with
THE SAME MARK 
The usual 'C' believed to be for Crucible is not present, and I have not previously seen the 'I', although it could be another letter partially ground away.
4# 3oz. was likely the pre-finish barrel weight.
In that there is essentially NO chance that Sanderson Bros., Tacony Iron Works, or another Crucible steel work was producing Twist rough forged tubes, the presence of the SB&Co mark would seem to be very strong evidence that Sanderson/Crucible served as the importer/agent/broker for the Belgian fluid steel tubes.
Or maybe it was Monday after St. Patrick's Day Sunday and someone with the barrel stamper was having a bad day
